This invention relates to a thioester type liquid crystal compound, a liquid crystal composition including the same and a liquid crystal display device including the liquid crystal composition.
Heretofore, various display principles have been applied to electrooptical display devices using liquid crystals and practical liquid crystal display devices have already been available commercially. Among them, TN type liquid crystal display devices using nematic liquid crystals with twisted alignment have widely been used in wrist watches, table-type electric computers, etc.
Recently, it is noticed that smectic liquid crystals show a light scattering phenomenon similar to the dynamic light scattering phenomenon of nematic liquid crystals and a display method applying the thermal and electrooptical effect is proposed. Such a liquid crystal display method is noticed as having memory function (e.g. U.S. Pat. No. 4,202,010).
According to such a method, when a liquid crystal layer is changed from a smectic phase to an isotropic liquid phase by heating an appropriate portion, there takes place an optically scattered state at the appropriate portion by subsequent rapid cooling. On the other hand, the non-heated portion of liquid crystal layer remains transparent due to the maintenance of uniform arranged structure. Thus, writing of images is conducted by forming scattering points in the transparent substrate. Further, selective erasure of a part of recorded image can be conducted as follows. That is, when an electric field is applied with higher than a certain value to the liquid crystal phase obtained by heating the smectic phase to the isotropic phase, the liquid crystal phase returns to the arranged smectic phase which is transparent. The transition from the smectic phase with a scattered structure to the smectic phase with an arranged structure can also be attained by applying a by far higher electric field than that mentioned above without heating.
But it is very important to drive even such a thermal address display element using smectic liquid crystals with a low voltage and a low power. Therefore, it has been required to find out liquid crystals and liquid crystal compositions satisfying such a requirement.
Generally speaking, in order to lower the threshold voltage (Vth) of a liquid crystal corresponding to an operation voltage of a liquid crystal display device, it is necessary to enlarge the dielectric anisotropy (.DELTA..epsilon.). Among known smectic liquid crystal, the compounds of the formulae: ##STR2## wherein R.sub.1 is an alkyl group having 8 to 12 carbon atoms, have relatively large positive dielectric anisotropy. But even such biphenyl type liquid crystal compounds are still insufficient in the dielectric anisotropy.